Method and apparatus for determining a polling interval in a network management system

ABSTRACT

A method and apparatus for determining a time interval for periodically polling a device on a network  1  is described. The method determines the most restrictive link on the path to the network device, the level of restrictiveness of a link being dependent on the nature of the link, such as the link speed and link type. The time interval between polling is determined based on the nature of the most restrictive link. Thus, the time interval can be determined according to the type and speed of the links between the polling device, which in a preferred embodiment is a network management station, and the polled device on the network  1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the retrieval of data by polling adevice on a network, and more particularly to a method for adjusting thepolling interval according to characteristics of the network.

2. Description of the Related Art

The present invention will be described in relation to the retrieval ofnetwork management data by a network management software application.However, the skilled person will appreciate that the invention can beused for many other purposes in which data is collected by periodicallypolling a device on a network.

Network management applications are commonly used to monitor theperformance of a network. For example, a network management applicationmay monitor network activity to determine that the devices and linksforming the network are operating efficiently, and are not undulycongested with network traffic. Monitoring typically includes “polling”network devices. Polling may involve performing tests for deviceactivity and service availability, and sending requests to managednetwork devices for management data indicative of device and linkoperation, at predetermined periodic intervals.

For example GB-A-2 350 035 describes a network management applicationwhich performs periodic polling of devices on a Local Area Network(LAN), and which retrieves network management data using the SimpleNetwork Management Protocol (SNMP). In accordance with the SNMPprotocol, the SNMP manager in the network management station, runningthe network management application, sends polling requests, at periodicintervals, to the SNMP agents of managed network devices on the networkto retrieve specified SNMP MIB data relating to characteristics, such astraffic activity or errors occurring at a particular port, forprocessing by the network management station. In addition, the networkmanagement station performs polling tests at periodic intervals,including sending ICMP Ping requests to each device on the network, andsending selected requests for services such as SMTP, NFS and DNS toservers, and monitoring the time taken to receive a response. If aresponse is not received in a predetermined time interval a “timeout” isrecorded which may indicate that the monitored device is not operatingcorrectly or there is a problem with a link between the monitored deviceand the network management station.

An example of a network management application which performs periodicpolling of network devices is the 3Com® Network Supervisor, availablefrom 3Com Corporation of Santa Clara, USA.

The polling interval for a particular request or test sent to aparticular network device is typically preset in the network managementapplication, and is dependent upon the monitored characteristic, thetype of device being polled etc. Typically the polling interval is inthe range of 30 seconds to 5 minutes, which, if a large number ofnetwork devices are being monitored, generates a significant quantity ofnetwork traffic that may cause congestion on the network. Moreover, iflinks between monitored network devices and the management stationrunning the network management application are relatively slow speedlinks, the delays in the monitoring traffic may cause problems.

For example, if a network administrator is running a network managementapplication at a location remote from the monitored network, for examplevia a modem connection (via the PSTN) or a Wide Area Network (WAN) link,the slow speed of the link may cause polling requests and pollingresponses to be delayed, thus resulting in the network managementapplication reporting “timeouts”. Such timeouts may give the networkadministrator a false impression about the state of the network bysuggesting that there are problems on the monitored network that do notexist.

In addition, the problem of congestion increases if a link between thenetwork management station and the monitored network is shared withother network users, as is the case with a WAN link, since thecongestion caused by the network management traffic may unacceptablyslow reduce the bandwidth available to other users, thus slowing downnetwork communications for other users.

Finally, another problem arises if the network management trafficutilised a “dial-on-demand” link, such as an ISDN line, in which thelink is made upon data being sent and broken a predetermined time afterthe link stops being used. In particular, polling may cause the link tobe continually made and broken, for example if the polling interval is 2minutes and the line is automatically disconnected after 60 seconds ofinactivity, which has undesirable cost implications and represents apoor use of the link.

In order to address these problems, the present invention provides amethod for adapting the polling interval for the monitoring of aparticular characteristic of a device on a network according to networkcharacteristics, and in particular according to the nature of the linkscarrying the monitoring requests and responses.

SUMMARY OF THE INVENTION

In accordance with a first aspect, the present invention provides amethod for determining a time interval for periodically polling a deviceon a network, the method comprising determining the most restrictivelink to the network device, the most restrictive link being dependent onthe nature of the link, and determining the time interval in accordancewith the nature of the most restrictive link.

By determining the polling interval according to the nature (e.g. typeand speed) of the most restrictive link to the polled network device,congestion due to polling traffic can be minimised (i.e. by increasingthe polling interval for slower and/or shared links), unnecessary use ofdial on demand links can be avoided (e.g. by increasing the pollinginterval), and more polling information can be gathered where link speedand bandwidth is available (i.e. by reducing the polling interval wherethe default polling interval is unnecessarily high).

Preferably, the method is performed by a polling device on a network,the polling device periodically polling a plurality of devices on thenetwork.

In a preferred embodiment the nature of the most restrictive link isdetermined using the steps of: determining the nature of the link of thelocal network to which the polling device is connected; determining thenature of the restrictive link to the subnet to which the polled deviceis connected, and comparing the nature of the determined links.

In accordance with a second aspect, the present invention provides acomputer program for carrying out a method in accordance with a firstaspect of the present invention.

In accordance with a third aspect, the present invention provides anetwork management apparatus for periodically polling devices on anetwork, the apparatus configured to determine a time interval forperiodically polling a device on a network by determining the mostrestrictive link to the network device, the apparatus being configuredto determine the time interval in accordance with the nature of the mostrestrictive link.

Further preferred features and advantages of the present invention willbe apparent from the following description and accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 illustrates an example of a typical network including a networkmanagement station performing periodic polling of network devicesaccording to the present invention, and

FIG. 2 is a flow diagram showing the steps performed by a computerprogram implementing a method according to a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a simple example of a Wide Area Network (WAN) 1 comprisinga plurality of Local Area Network (LAN) subnets 1A, 1B and 1C (only LANAis shown in detail; the skilled person will appreciated that LANs B andC have similar configurations). Each subnet comprises a plurality ofnetwork devices comprising a router 9 connecting the subnet to the WANby means of WAN links, and one or more core SNMP managed network devices7 such as switches or hubs, and unmanaged workstations and PCs 3, allconnected by media links 5 (which may be coaxial, twisted pair or fibreoptic cable, or wireless links). As well known in the art, each managednetwork device includes an SNMP agent capable of monitoring data packetsand storing MIB (Management Information Base) data which can beretrieved by an SNMP manager, in a network management station, inresponse to SNMP requests.

A network management station 3A includes a network managementapplication which monitors the network by polling network devices byperforming tests and sending requests to retrieve SNMP MIB data asdescribed above.

The time interval between polling a particular network device istypically set to a default value in the network management application,the default value being dependent upon the characteristic of the networkdevice being monitored. The default value for each characteristic isdetermined by the application vendor based on typical networkconfigurations. Thus, the polling interval is not tailored to theparticular network or networks being monitored by the network managementapplication.

The method and apparatus of the present invention adjusts the pollinginterval for each monitored characteristic and for each device accordingto the network, specifically according to the nature (type and speed(data rate/bandwidth)) of the links between the network managementstation 3A, running the network management application, and themonitored network.

FIG. 2 illustrates a method for adjusting a polling interval inaccordance with a preferred embodiment of the present invention. Inorder to implement the method of the present invention, all that isrequired is a list of the network devices to be polled, which may bestored in memory as a list of the IP addresses (or equivalent) of thenetwork devices. It will be appreciated that in some embodiments, moredetailed information about the topology of the network may be available.

The method is preferably implemented in the form of a computer programforming part of a network management application. The computer programis preferably provided in the form of a computer readable medium such asa magnetic or optical disk, for loading onto the network managementstation 3A, or may be downloaded as a carrier wave over a suitablecommunications medium from a website, for example. It will beappreciated that the method may also be implemented in other forms suchas hardware.

The program starts at step 110 by determining the nature of the link ofthe local network on which the network management station 3A, runningthe network management application, is located. In particular, thenetwork management application determines the type and speed of thenetwork link to which the management station 3A is connected. This canbe done using conventional techniques, well known in the art.

For example, the type and speed of the link can be determined from SNMPMIB data retrieved from the SNMP managed device to which the networkmanagement station 3A is connected. In particular, the standard SNMP“ifTable” in MIB II includes the variables “ifType” and “ifSpeed”, whichcan be read for the particular link concerned. The “ifType” value isdefined as “the type of interface, distinguished according to thephysical/link protocol(s) immediately ‘below’ the network layer in theprotocol stack” thus indicates the type of link. The “ifSpeed” value isdefined as “an estimate of the interface's current bandwidth in bits persecond” and indicates the operating speed of the link.

At step 120, the program then selects a first subnet, e.g. LANA, of themonitored network (WAN 1). The selection of a subnet can be performed onthe basis of the IP addresses of the network devices to be polled. Inparticular, the program can identify a subnet from the “subnet number”within the IP addresses of the network devices.

At step 130, the program determines the path between the networkmanagement station 3A and the relevant subnet 1A (LANA). This isperformed using conventional techniques. For example, the path may bedetermined using network topology information (if available) or by usingthe well known ICMP-based “TraceRoute” technique which finds routers onthe path to the subnet 1A. In the illustrated embodiment, the determinedpath to the subnet only includes the links between LANs (i.e. the linksconnecting all the routers between the network management station andthe subnet). It is not generally necessary to determine the unique pathsto individual network devices on the subnet, since the links connectingdevices on a subnet (LAN) are generally short and high speed, and sorarely introduce delays to polling traffic. Nevertheless, in otherembodiments, it may be desirable to determine the paths to selected orall polled devices, including the subnet (LAN) links.

At step 140, the program determines the nature, specifically the speedand optionally the type, of each link on the path to the subnetdetermined in step 130. The speed and type of each link can bedetermined using conventional techniques, for instance using SNMP asdescribed above in relation to step 110. Alternatively, the type andspeed of one or more of the links may be obtained from the Telnet or webinterface of a device connected to the link.

At step 150, the program identifies the most “restrictive” link of thelinks in the path to the subnet 1A based on the speed, and optionallythe type, of links in the path, determined at step 140. The mostrestrictive link may be determined simply by comparing the speed (datarate) of the links in the path to the subnet, the most restrictive linkbeing the link with the lowest link speed. Alternatively, in morecomplex embodiments, a “restrictiveness value” may be assigned for eachtype and speed of link, and the restrictiveness values for the links inthe path compared in order to determine the most restrictive link. Thus,for example, a POTS link may have a highest restrictiveness value of,say 10, indicting the most restrictive link type and speed, and a dialon demand ISDN link may similarly have a high restrictiveness value of,say 9. In contrast, a high speed/bandwidth link may have relatively lowrestrictiveness value indicating a least restrictive link. A wirelesslink may have a moderate restrictiveness value, according to its typeand speed, and so on.

At step 160 the program compares the speed and type of the mostrestrictive link, as determined at step 150, with the speed and type ofthe local link, and determines which is the most restrictive. Again,this may be performed by simply comparing the speed of the links, or bycomparing “restrictiveness values” as described above.

If step 160 determines that the local link is the most restrictive link,the program proceeds to step 170 and determines the polling interval(s)according to the speed and type of the local link, as described below.

Alternatively, if step 160 determines that the local link is not themost restrictive link, the program proceeds to step 180 and determinesthe polling interval(s) according to the speed and type of the mostrestrictive link in the path, as determined at step 150, as describedbelow.

At step 190, the program determines whether there are any more subnetsto be considered by considering the list of IP addresses of networkdevices to be polled.

If there are more subnets to be considered, the program returns to step120 and selects the next subnet. The program then proceeds with steps130 to 170/180 for the new subnet.

The program continues until step 190 determined that there are no moresubnets to consider and then ends.

The determination of the polling interval, at step 170 or step 180according to the appropriate (most restrictive) link is performed asfollows.

The polling interval needs to be chosen in order to limit the proportionof the bandwidth used by monitoring traffic over a given time period.The proportion of the bandwidth that may be used for monitoring trafficwill depend on the nature of the most restrictive link. Thus for a highspeed link, with relatively little other traffic, a greater proportionof the bandwidth may be appropriate, whereas a low speed shared linkcarrying large quantities of other traffic a lower proportion of thebandwidth. The polling interval is then determined based on theproportion of the bandwidth that is available for monitoring traffic,based on these factors.

For example, say for a particular type of link, up to 10% of thebandwidth is available for monitoring traffic. Then the threshold forlink utilisation for monitoring traffic is 10% or 0.1.

The utilisation of the link by monitoring traffic is determined as anaverage utilisation over a time period, expressed as a percentage of theoverall bandwidth.

Thus, the utilisation by monitoring traffic, U, is defined by theequation:

$U = \frac{N/P}{B}$where:

-   B=the bandwidth of the link (in b/s)-   N=the number of data bits sent and received per poll-   P=polling interval (in s)

Since the bandwidth B is a fixed value (i.e. the link speed 64 kb/s, 10Mb/s etc) and the number of data bits sent per poll N can be determinedby the polling device, the polling interval P can be calculated so thatthe utilisation U is less that the threshold U_(T)(e.g. 0.1 for 10%)using the equation:

$P \geq \frac{N}{U_{T}B}$

For dial on demand link types such as ISDN, for which a connection hasto be established, the “period of inactivity”, which leads to the linkbeing broken, may also be taken into consideration when determining thepolling interval. The period of inactivity which causes such a link tobe broken may be entered manually by a network administrator, ordetermined by other conventional methods. The polling interval may beincreased or reduced according to the period of inactivity and theavailable bandwidth. Alternatively, the method may determine thatpolling interval based on the link speed, and maintain the link in aconstantly active state, by sending relatively small data packets on thelink at periodic intervals less than the period of inactivity associatedwith the link.

Accordingly, the present invention enables the adaptation of the pollinginterval according to the polling environment. The present invention maybe used to dynamically modify the polling interval, for each polleddevice (or for each polled device on a subnet), by monitoring the amountof polling traffic, particularly polling traffic sent on the mostrestrictive link or links to the device, in order to maintain the volumepolling traffic below predetermined acceptable threshold levels.

As the skilled person will appreciate, various modifications and changesmay be made to the described embodiments. It is intended to include allsuch variations, modifications and equivalents which fall within thespirit and scope of the present invention as defined in the accompanyingclaims.

1. A method for determining a time interval for periodically polling anetwork device on a network, the method comprising determining a mostrestrictive link in a path to the network device from a second device,the restrictiveness of a link being dependent on the nature of the linkdetermining the time interval according to the nature of the mostrestrictive link, and polling at a rate specified by the time interval,wherein the step of determining a most restrictive link to the networkdevice comprises: determining the nature of at least the links betweenrouters on a path to the network device, and determining the mostrestrictive link according to the determined nature of the links.
 2. Amethod as claimed in claim 1, wherein the step of determining the mostrestrictive link comprises, for each link, determining a restrictivenessvalue based on the determined nature of the link; comparing thedetermined restrictiveness values, and determining as the mostrestrictive link the link having a highest or lowest restrictivenessvalue representing the most restrictive link.
 3. A method as claimed inclaim 1, wherein the step of determining the most restrictive linkfurther includes identifying at least the links between routers on apath between the polling device and a subnet to which the network deviceis connected.
 4. A method for determining a time interval forperiodically polling a network device on a network, the methodcomprising determining a most restrictive link in a path to the networkdevice from a second device, the restrictiveness of a link beingdependent on the nature of the link, determining the time intervalaccording to the nature of the most restrictive link, and polling at arate specified by the time interval, wherein the time interval isdetermined such that data traffic generated by polling is less than apredetermined proportion of the bandwidth of the most restrictive link.5. A method as claimed in claim 4, wherein the step of determining thetime interval is performed using the equation: $P \geq \frac{N}{U_{T}B}$where: P=the time interval (in s); N=the number of data bits associatedwith a poll; U_(T)=the proportion of the bandwidth that may be used forpolling traffic, and B=the bandwidth of the most restrictive link (inb/s).
 6. An apparatus for determining a time interval for periodicallypolling a network device on a network, the apparatus determining themost restrictive link in a path from a second device to the networkdevice, the level of restrictiveness of a link being dependent on thenature of the link, the apparatus determining the time interval inaccordance with the nature of the determined most restrictive link, andpolling at a rate specified by the time interval, wherein the mostrestrictive link to the network device is determined by determining thenature of at least the links between routers on a path to the networkdevice, and determining the most restrictive link according to thedetermined nature of the links.
 7. An apparatus as claimed in claim 6,further comprising a memory for storing restrictiveness values, eachrestrictiveness value corresponding to a nature of link, wherein, theapparatus is further configured to retrieve from memory arestrictiveness value for each link on a path to the network device,using the determined nature of the link, and to determine the mostrestrictive link by comparing the restrictiveness values for said links.